Known examples of LAN configurations include a LAN connected to a fixed backbone network via a router of the kind described in document (1) "Interconnections: Bridges and Routers" by Radia Perlman, Addison-Wesley, 1992 (this LAN is referred to as a "backbone-connected LAN"), and a LAN temporarily set up solely of terminals and not connected to a backbone network via a router, as described in document (2) "Routing in Ad Hoc Networks of Mobile Hosts" by David B. Johnson, Proceedings of the Workshop on Mobile Computing Systems and Applications, pp. 158.about.163, IEEE Computer Society, Santa Cruz, Calif., December 1994 (this LAN is referred to as an "ad hoc LAN").
These LANs have different features and the circumstances in which they are used differ in the following way: In the case of the backbone-connected LAN, communication is carried out among terminals and servers connected to other LANs mutually connected by a fixed backbone network. A backbone-connected LAN is suited to a regularly performed operation such as the sending and receiving of electronic mail for a daily business.
An ad hoc LAN, on the other hand, brings together terminals having a communication function and only these terminals communicate with one another. This LAN is useful in a scenario such as a conference in which the participants gather together bringing their terminals with them.
The configurations of a backbone-connected LAN and ad hoc LAN are illustrated in FIGS. 8 and 9, respectively.
As shown in FIG. 8, backbone-connected LANs 1010, 1020 are connected to a backbone network 1000. Though only two backbone-connected LANs are depicted in FIG. 8, there are instances where three or more of these LANs are thus connected.
The internal construction of the backbone-connected LANs will be described taking the LAN 1010 as an example.
As shown in FIG. 8, the backbone-connected LAN 1010 is connected to the backbone network 1000 via a router 1005. Conceivable transmission media are a physical wire as at 1007 or a wireless link as at 1006.
Examples of terminals used within the backbone-connected LAN are a wired terminal 1004 connected via the wire transmission medium 1007 and wireless terminals 1001, 1002 connected via the wireless transmission medium 1006. The wireless terminals 1001, 1002 are capable of communicating with the wired terminal 1004 and router 1005, which are connected to the wired medium 1007, via a wireless base station 1003.
Further, the terminals in the backbone-connected LAN 1010 communicate with terminals connected to another backbone-connected LAN, such as the backbone-connected LAN 1020, via the router 1005 and backbone network 1000.
The ad hoc LAN, on the other hand, is constructed as shown in FIG. 9. FIG. 9 schematically illustrates the manner in which two ad hoc LANs 1100 and 1110 exist. Though only two ad hoc LANs are depicted in FIG. 9, there are instances where three or more of these LANs may exist.
As shown in FIG. 9, the ad hoc LANs 1100 and 1110 include three wireless terminals 1101, 1102, 1103 and 1111, 1112, 1113, respectively. The wireless terminals within each ad hoc LAN communicate with one another via wireless transmission media 1104 and 1114, respectively, but there is no communication between the ad hoc LANs. Though FIG. 9 illustrates an arrangement in which the ad hoc LANs use wireless transmission media, it is possible to adopt an arrangement in which only wired transmission media are used or one in which both wired and wireless transmission media are employed.
A method relying upon connection by a router and a method relying upon connection by a bridge are known as methods of connecting different LANs. For example, see the document (1) above ("Interconnections: Bridges and Routers" by Radia Perlman, Addison-Wesley, 1992).
Such arrangements are illustrated in FIGS. 10 and 11, which show the manner in which a terminal A (1121) and a terminal B (1131) communicate with each other in a case where different LANs comprising respective transmission media 1120, 1130 are interconnected via a relay node 1125 (FIG. 10) or 1126 (FIG. 11).
FIG. 10 illustrates connection by router, in which the relay node 1125 functions as the router. In this case the relay node 1125 is such that processing for transferring packets used in communication between the terminal A and the terminal B is executed in the network layer.
FIG. 11, on the other hand, illustrates connection by bridge, in which the relay node 1126 functions as the bridge. In this case the relay node 1126 is such that processing for transferring packets used in communication between the terminal A and the terminal B is executed in the data link layer.
The router arrangement is implemented using addresses in the network layer. Though transfer processing presents a heavy load, flexible control is possible.
With the bridge arrangement, on the other hand, only simple transfer control can be carried out but the transfer processing is simple.
Multicast communication is known as a communication scheme in which bandwidth is utilized effectively. According to multicast communication, a plurality of parties are defined as a group, and in case of communication within the group, data is not transmitted to the individual parties but is instead multicast to the defined group by a single transmission. For a description of multicasting, see document (3) "Mbone: Interactive Multimedia on the Internet" by Vinay Kimar, New Riders Publishing, 1995 etc.
Multicasting is implemented by defining a multicast address for each multicast group. In a case where data is transmitted to a certain multicast group, the data is transmitted with the multicast address of this group serving as the destination address. A terminal participating in the multicast group receives the data that has been transmitted using the multicast address of the group as the destination of the transmission. The packet format used in such multicast communication is as shown in FIG. 14. As shown in FIG. 14, the format includes a network layer header 1300 and higher layer data 1301. The network layer header 1300 includes a multicast address as the destination and a transmitting terminal address as the address of the transmitting party.